The present invention relates to an optical single sideband transmitter; and, more particularly, the invention relates to an optical single sideband modulation (SSB) method, which is used for bandwidth reduction of an RZ (return-to-zero) modulated optical signal as provided in optical information communication based on use of an optical fiber, and to the structure of an optical transmitter based on that method.
The wavelength-division multiplexing (WDM) method, which provides optical information transmission by multiplexing a plurality of optical signals having to different wavelengths within an optical fiber, is an extremely effective means for increasing the capacity of optical fiber communication.
In recent years, WDM (wavelength-division multiplexing) optical transmission equipment, having more than 100 wavelengths and a total transmission rate of higher than 1 Tbps, has been commercialized. Further, a transmission system having a tenfold number of wavelengths and a transmission rate that is 0.10 times higher is being experimentally studied for implementation. An extremely wide frequency (wavelength) bandwidth is required for such large-capacity information transmission. Its upper limit is determined by the optical fiber's low-loss wavelength bandwidth and the amplification wavelength bandwidth of an optical amplifier, such as an erbium-doped fiber amplifier (EDFA) or other rare-earth doped fiber amplifier for optical signal relay/amplification in the middle of a transmission path, semiconductor optical amplifier, or optical fiber Raman amplifier. The widely used C-band EDFA wavelength bandwidth is 30 nm, which is between 1530 nm and 1560 nm and is equivalent to a frequency width of about 3.8 THz. Although the use of an L-band optical amplifier or Raman amplifier can increase the wavelength bandwidth several times, it lowers the pumping efficiency, which results, for instance, in a cost increase and a deterioration in the optical amplifier performance.
Another method of making effective use of the above-mentioned limited wavelength bandwidth to increase the transmission rate is to reduce the optical signal's signal bandwidth and arrange optical signals (channels) more densely, so as to increase the optical signal frequency (wavelength) density. One example of this method is the optical single sideband modulation method used by the present invention. Although this method is widely used for radio communication, it has not been commercialized for optical fiber communication, and basic investigations concerning this method are currently conducted, for example, mainly by scientific societies. Typical techniques proposed for this method are optical single sideband (SSB) modulation and optical vestigial sideband (VSB) modulation. The SSB technique performs signal processing within an electrical region to directly generate an optical single sideband signal. The VSB technique subjects a double sideband optical signal to optical filtering to extract one of two sidebands and remove the other for the purpose of reducing the signal bandwidth to one-half.
Meanwhile, two optical signal digital intensity modulation methods are normally used: NRZ (nonreturn-to-zero) and RZ (return-to-zero). NRZ modulation has the advantage that it facilitates the optical modulator configuration. RZ modulation, on the other hand, is advantageous in that it is highly immune to optical fiber nonlinear effects and polarization mode dispersion, and it os characterized by the fact that it is unlikely to suffer from waveform degradation even when it is subjected to narrow-band optical filtering. In recent years, therefore, RZ modulation has been highlighted in the field of large-capacity wavelength-division multiplexing. However, RZ modulation generally requires a greater signal bandwidth than NRZ modulation, and it cannot increase the frequency density with ease. In that sense, RZ modulation is disadvantageous in terms of spectral efficiency.
The present invention deals with a technology that extracts one of two RZ signal sidebands so as to reduce the optical signal bandwidth to half. In the examples given below, the applications of two conventional single sideband modulation methods to the RZ modulation method are described to point out the problems encountered with the conventional methods.
Studies are conducted so as to apply the conventional SSB modulation method mainly to the NRZ modulation method. FIG. 4 shows an example in which the conventional SSB method is applied to an RZ modulated optical transmitter. In this example, NRZ data signals (Q and Q′), which are opposite in phase with each other, are first supplied to RZ signal generator circuits 112-1, 112-2, respectively, and converted to RZ electrical signals that are opposite in phase with each other. The RZ signal generator circuits can be implemented by, for example, gating a clock signal with a data signal. Within an SSB signal generator circuit 113, a 90-degree phase shifter 110 shifts the phase of inverted signal Q′ (−180 degrees) by 90 degrees to generate a data signal having a phase angle of −90 degrees. An addition circuit 111-1 subjects the resulting data signal and the noninverted signal Q (0 degrees) to vector addition to generate a drive signal 106-1 having a phase angle of 45 degrees. Meanwhile, an addition circuit 111-2 subjects the above-mentioned signal having a phase angle of −90 degrees and the inverted signal Q′ to vector addition to generate a drive signal 106-2 having a phase angle of −135 degrees. The two drive signals 106-1, 106-2 are 90 degrees out of phase from each other within the entire frequency range. When they are applied to the electrodes of a dual-arm drive Mach-Zehnder optical modulator, the laser light (wavelength: A), which is output from a laser source 100, can be intensity-modulated to generate an RZ single sideband signal. Note that a phase shifter 102 compensates for the path length difference between the two drive signals 106-1, 106-2, which may be caused by a manufacturing error or the like. FIG. 5 shows a typical optical spectrum of a single sideband signal obtained in the above-mentioned manner. The original signal wavelength is λ. Thanks to data signal modulation, the optical signal spectrum of a normal RZ signal expands by a width of Rb in both directions. In the example shown in the figure, however, the sideband intensity on the long wavelength side is suppressed by more than 10 dB due to the SSB modulation effects. Theoretically, 100% intensity suppression is achievable.
However, since this method entails high-frequency signal processing, significant waveform deterioration occurs. As a result, the symmetry between the two drive signals 106-1, 106-2 is destroyed so that perfect sideband suppression is difficult to achieve. In most cases, the degree of single sideband suppression is about 10 dB, as shown in FIG. 5. In a practical WDM transmission, however, 20 dB or a higher degree of suppression is required to prevent signal quality deterioration, which can be caused by the interference from neighboring channels. This is the reason why this method cannot be readily implemented. Further, this method is at a disadvantage in that it requires a complicated modulation circuit, which raises the cost of the transmitter. Particularly, the RZ signal has about two times the electrical signal bandwidth of the NRZ signal and, therefore, entails higher-frequency signal processing, making expensive high-frequency component parts necessary.
Mathematically, the 90-degree phase shifter 110 is a circuit that performs a Hilbert transform. At present, however, it is extremely difficult to fabricate a circuit that performs a Hilbert transform over the entire frequency range. Experimentally, a 90-degree hybrid or other microwave component may be used as a substitute for approximation. In this instance, however, the signal's low-frequency component is lost. Consequently, digital signals used for normal optical fiber communication cannot be subjected to SSB conversion, which is a major problem for practical use.
FIG. 6 shows a typical RZ modulation single sideband optical transmitter to which the conventional VSB method is applied. The light output from a laser source 100 enters an optical pulse generation optical modulator 120. This optical modulator is driven by a sine wave clock signal (frequency: Rb). As a result, an intensity-modulated optical pulse train, having a repetition period of Rb, is output to point A, as seen in the figure.
FIGS. 7A through 7D show the optical signal spectra at the points shown in FIG. 6. FIG. 7A shows an optical spectrum at point A. The optical pulse train spectrum has two sidebands, which are positioned on either side of and are spaced apart by Rb from a central carrier having a wavelength of A. The optical pulse train is then supplied to an NRZ optical modulator 103, which is driven by an NRZ electrical information signal having a bit rate of Rb, gated, and converted to an RZ optical signal. FIG. 7B shows the RZ optical signal's spectrum, which is broadened by information signal modulation. Subsequently, the optical signal is filtered by a VSB narrow-band optical filter 121, converted to a vestigial sideband signal, and output from optical fiber 105. FIG. 7C shows the transmissive characteristics of the VSB narrow-band optical filter 121. The figure indicates that transmission occurs through the signal's only one sideband (short-wavelength side), and that the bandwidth is about half the signal spectrum, and further that the center wavelength is shifted away from a center frequency of λ and toward the short-wavelength side. Therefore, the long-wavelength side sideband of the output optical signal at point C is suppressed, as shown in FIG. 7D, so that the bandwidth is reduced to about one-half.
The VSB optical transmitter based on the conventional technology, as described above, has many problems, as will be explained below. First of all, when this method is used, an optical filter removes one of two sidebands of a double sideband optical signal and part of the central carrier. Therefore, the optical loss is significant (at least 3 dB). In addition, the received signal amplitude lowers, thereby to degrade the receiving sensitivity due to the loss of the central carrier.
Further, the output optical signal's spectrum shape is determined by the shape of the VSB narrow-band optical filter. Therefore, if there is an error in the optical filter shape or bandwidth, waveform deterioration results, thereby reducing the receiving sensitivity. It is extremely difficult to control these factors on the order of several gigahertz (approximately one-tenth of a signal bit rate). It is also necessary that the difference between the optical signal wavelength and narrow-band filter transmission band center wavelength be set with extremely high accuracy. If both of these wavelengths are in error, receiving sensitivity deterioration, crosstalk between neighboring wavelengths, or other significant performance deterioration results. Particularly, the wavelength stabilization of an optical signal to a position spaced away from the optical filter center is susceptible to disturbances, such as intensity variations in an input optical signal or transmission characteristics changes with time. Therefore, a control error is likely to occur.